Remoting fixes for large workloads

Lots of changes in this PR:

1. Added a new TimeServer / TimeClient example - ported from Helios. Designed to make it easy to test large workloads for Akka.Remote and multi-client stuff with Akka.Remote.
1. Fixed a number of bugs with the StashProvider and `UntypedActorWithStash` 
1. Drastically improved Helios performance and reliability for high speed writes and reads
1. Added tons of reliability improvements to EndpointManager and Endpoint.cs for cleanly terminating connections / pruning gated connections and so forth. 

I have more work to do on fixing up some reliability issues with Akka.Remote; testing it under large production loads at the moment.